At present there are no standards to follow in the design of the firefighting water supply system of extralong submarine Metro tunnels. In the paper, the causes for and characteristics of the fires in extralong submarine Metro tunnels are analyzed on basis of the design of the firefighting water supply system of extralong submarine Metro tunnels in Qingdao and Xiamen; typical schemes, i.e., fire hydrant system, AFFF (aqueous film forming foam) system and automatic waterspray fireextinguishing system, are proposed for the extralong submarine Metro tunnels; some key technical issues related to the design of the water supply of the firefighting system of extralong submarine Metro tunnels, including the scheme comparison and design considerations, are discussed; it is proposed that "fire hydrant system + high pressure water mist system installed in the connection galleries" should be adopted; it is pointed out that special attention should be paid to the material and structural safety of the pipes in the design. 